It's a question that's baffled evolutionary theorists for decades: if survival of the fittest is the rule, how have the genes that contribute to serious, debilitating mental disorders survived?

It's been shown that people who suffer from schizophrenia, autism, anorexia and other disorders are less likely to have children. And yet, the genes that cause these disorders aren't going away. In fact, some of the disorders appear to be becoming more common. Evolutionary theory wouldn't predict that.

Scientists have a few theories that attempt to explain this paradox.

One is that the genetic mutations that cause these disorders occurred relatively recently, so not enough generations have passed to allow the evolutionary process to weed them out.

Another theory is that the genetic mutations that cause a disorder in one person somehow make that person's sibling more likely to have children. In a situation like that, the mutation offers a net benefit to a person's family.

A team of Swedish and British scientists recently tested these theories by comparing the rates at which people suffering from mental illness have kids to those of their siblings. The data came from a medical database of more than 2 million Swedes.

The researchers found that the siblings of people who suffer from schizophrenia, autism and anorexia had on average the same or fewer children than the general public, which would seem to confirm the first theory. But they also found that the siblings of people who suffer from depression or substance abuse had significantly more children than the general public, an outcome more in line with the second theory.

Q: You say at the beginning of your paper that "psychiatric disorders have long puzzled researchers by defying the expectations of natural selection." Why?

A: It's particularly the case with schizophrenia, which in this paper and in many other papers has been shown to be a disorder that drastically reduces your fecundity — the number of kids you have. It's often referred to as reduced fertility but, strictly speaking, people with schizophrenia aren't infertile. It's just that they're less often likely to find a partner and have kids.

Schizophrenia is estimated to have a heritability of around 80 percent. Same is true for autism. So if these disorders are very heavily influenced by genes, but the people who have the disorders are less likely to pass on their genes, why aren't the disorders becoming less common in the population?

Q: What are some of the theories as to why this might be going on?

A: There are other gene disorders that have selection pressures against them, but are maintained in the population. A brilliant example of that is sickle cell disease. If you have sickle cell disease, the chances are if it's untreated it's going to kill you before you reach early adult life. Whereas, if you have the sickle cell trait — which is to say, you have one copy of the gene, not two copies — it protects you against malaria if you happen to live in an area where malaria is rife. So there's a selective disadvantage to having the disease, but there's a selective advantage to having the trait.

Q: Your study looked at not just people who were affected by psychiatric disorders, but also their siblings. Why?

A: The hypothesis would be that the relatives of the people who have the disorder, who don't actually have the disorder themselves, are compensating by having more children. I mean, not deliberately compensating by going out and having more children, but there's just something about their makeup that makes them have more kids.

So that's essentially what we were testing in this paper. We were looking at the fecundity of schizophrenics, which we found to be low, as was the fecundity of people with autism. The question is, do their relatives actually make up for this by having more kids because they're advantaged in some way? And the answer is no in the case of schizophrenia, but yes in the case of depression.

Q: If I'm someone who is the sibling of someone who has a psychiatric disorder, what do I need to know? Do I need to think twice about having biological children?

A: You need to know you'll have an increased risk of getting the disorder yourself compared with the general population. The risk in siblings is increased, but it's not increased so dramatically that it ought to stop you from having kids.

These aren't single-gene disorders. These are complex disorders where being a relative is just a risk factor, not a certainty factor.

Q: So ultimately, if I'm the sibling of someone who has one of these disorders, I should be aware of the risks, but it's not something that would make me say "I'm not going to have children."

A: It shouldn't make you say that. And if you are particularly concerned about it and you have more than one relative affected by it in the family, it might be worth seeking [genetic] counseling from an expert who knows what the risks are.

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